A low temperature co-fired ceramic (LTCC) material system consists of a low firing temperature ceramic with multiple layers of high conductivity metals (e.g., gold, silver, and copper) used in a thin film processes. This technology allows for low temperature (<1000° C.) processing of three dimensional packages and the use of conventional chip and wire technologies for the fabrication of various LTCC packages.
LTCC is a glass matrix ceramic with a crystalline filler added or formed from the glass during the firing process. The crystalline filler is added to control thermal expansion characteristics, to control the densification behavior of the LTCC, and to achieve specific electrical performance.
The development of LTCC technology has generated an increasing interest in multi-layer bandpass filters that meet the challenge of size, performance and cost requirements. The miniaturization of the LTCC filters expanded with the development of DuPont's GreenTape™ 9K7, a low loss material for high frequency applications. GreenTape™ provides a co-fireable system of gold, silver, and resistive components having low loss properties in excess of 100 GHz. However, new design concepts are still needed to exploit the material fully.
A simple stripline filter consists of three layers of conductors. The internal conductor is typically referred to as the “hot” conductor and the other two conductors, connected at signal ground, are typically referred to as “cold” or “ground” conductors. The “hot” conductor is embedded in an isotropic dielectric that completely surrounds the “hot” conductor.
The performance of microwave components in electronic systems is currently limited by increasingly difficult requirements on performance, size, weight, and power handling. Microwave filters comprise a large fraction of a module's space, while conventional miniaturized filters still suffer from high losses and degraded performance. Some current designs partially fill this gap but do not have a high enough Q-factor to achieve narrow bandwidths needed for high order Nyquist filtering applications or the low insertion losses needed for applications before any amplifier in an RF receiver or for enabling direct digital sampling of Radar.
The LTCC coupled stripline resonator filters proposed herein, for use as bandpass filters, are versatile and can be implemented with combline topology or with interdigital topology. The filter bandwidths of the present disclosure range from about 0.3 GHz to about 4.5 GHz. This bandwidth can be increased with an increase in 9K7 tape thickness. The frequency operation of the present filters is up into the high millimeter-wave (MMW) region. The millimeter-wave region of the electromagnetic spectrum is generally understood to have a wavelength from about 10 millimeters to about 1 millimeter. Millimeter waves are longer than infrared waves or x-rays, and shorter than radio waves or microwaves. The millimeter-wave region of the electromagnetic spectrum corresponds to radio band frequencies of about 30 GHz to about 300 GHz and may also be referred to as the Extremely High Frequency (EHF) range.